Overall power efficiency is important in RF communications equipment. Higher efficiency designs are smaller, cheaper, and generate less heat than lower efficiency designs. In battery powered RF communications equipment, such as mobile terminals, high overall power efficiency is even more important to maximize battery life and minimize the size and weight of the equipment. An RF power amplifier is one of the highest power consuming devices in a mobile terminal. The RF power amplifier is used in an RF transmitter to provide the RF signal for transmission, which is typically sent to an antenna. To maximize the power efficiency of the RF power amplifier, the input impedance to the antenna circuitry may need to be transformed to present an impedance to the RF power amplifier that optimizes RF power transfer to the antenna.
Mobile terminals, such as cell phones, wireless personal digital assistants, or the like, may be carried by a user during RF communications. An antenna of such a mobile terminal may come into contact with the user's body or other objects, which may significantly change the input impedance to the antenna and its associated circuitry. Such impedance changes may increase the magnitude of reflected signals due to impedance mismatches between the RF power amplifier, the antenna, and the antenna circuitry. The reflected signals may cause variations in the voltage standing wave ratio (VSWR) of the antenna. VSWR changes may reduce the net RF power transferred from the RF power amplifier to the antenna. One approach to reduce the effects of VSWR on net RF power transfer is a quadrature power amplifier.
A quadrature power amplifier has two parallel RF signal paths. One RF signal is phase-shifted 90 degrees from the other RF signal. After the final power amplifier stage, a phase-shift is applied to phase align and combine the two RF signals into a single RF signal. The phase-shifting and combining tends to reduce the impact of reflected signals on net RF power transfer to the antenna. Thus, there is a need for a quadrature power amplifier with a quadrature combiner and an impedance transformation circuit to optimize net RF power transfer to an antenna and improve overall power efficiency.